1. Field of the Invention
This invention relates to a brake control system for a four-wheel drive vehicle having a four-wheel drive mechanism and an antiskid brake system.
2. Description of the Prior Art
There has been known an antiskid brake system in which the hydraulic pressure to the brakes is controlled to limit the factor of skid of the wheels to thereby ensure the most efficient braking when the brakes are applied during running. Generally, the most efficient braking is obtained when the factor of skid of the wheels is near 20%. Such an antiskid brake system has been put into practice for two-wheel drive vehicles. For example, in Japanese Unexamined Patent Publication No. 57(1982)-7747, there is disclosed an antiskid brake system for a two-wheel drive vehicle in which the hydraulic pressure to the brakes is controlled according to the deceleration of the wheel speed to prevent the wheels from being locked. However, the antiskid brake system has not been put into practice for four-wheel drive vehicles.
This may be because of the following reason. In the antiskid brake system, the factor of skid of the wheels is obtained through the actual vehicle speed determined by detecting the rotational speeds of the four wheels and by comparing the rotational speed of skidding wheels with the rotational speed of the wheels which are not skidding. In the case of the four-wheel drive vehicles, the four wheels are mechanically drivingly connected with each other and the four wheels begin to skid simultaneously. This makes difficult determination of the actual vehicle speed. Though it may be theoretically possible to determine the actual vehicle speed by use of sound locator or the like, this approach is not practical at present in view of the manufacturing cost and the like.
The conventional antiskid brake system may be applied to the four-wheel drive vehicle by changing four-wheel drive to two-wheel drive when the brakes are applied. (Though not intended to be associated with the antiskid brake system, a four-wheel drive vehicle in which the front wheels and the rear wheels are disconnected from each other when the brakes are applied is disclosed in UK Patent Application GB 2 139 972A.) In this case, application of the brakes can be detected by use of a switch to be actuated in response to operation of the brake pedal, a switch to be actuated according to the brake fluid pressure or the like. However, if the drive is changed from four-wheel drive to two-wheel drive every time the brake pedal is pushed down, change of the drive between four-wheel drive and two-wheel drive is effected even when the brake pedal is lightly pushed down and is frequently effected when the brake pedal is repeatedly pushed down, thereby adversely affecting the running performance of the vehicle.